1. Field of the Invention
The present invention relates to a functional device having a functional layer, such as a thin film transistor, a dielectric capacitor, or a solar battery, and a method of manufacturing the same.
2. Description of the Related Art
Since the pn junction of a hydrogenated amorphous silicon was developed in 1976, the hydrogenated amorphous silicon has been being actively studied. The hydrogenated amorphous silicon has a structure in which a dangling bond in a network made of silicon is terminated by hydrogen or fluorine, and its film can be formed at a low temperature-equal to or lower than 300xc2x0 C. Consequently, the film can be formed on a cheap glass substrate. A study is being made to apply the hydrogenated amorphous silicon to a functional device such as a thin film transistor (TFT), a solar cell, or an optical sensor.
However, when the hydrogenated amorphous silicon is used as it is, in the case of a TFT, only carrier mobility as low as about 0.1 to 0.5 cm2/Vxc2x7s can be obtained. In the case of a solar cell, there are drawbacks such that doping efficiency is lower as compared with the case of using polycrystalline silicon (polysilicon), and photoelectric conversion efficiency deteriorates due to an increase in series resistance. In recent years, a method of solving the problems by irradiating amorphous silicon formed on a glass substrate with an energy beam such as excimer laser beam so as to be crystallized is being studied. Recently, crystallization of not only semiconductors but also oxides performed by irradiation of an energy beam is also being studied.
In the functional devices, substrate for supporting a functional layer made of silicon, oxide, or the like is required to be light, shock-resistant, and flexible so as not to be broken when some stress is applied. Conventionally, a silicon substrate, a glass substrate, or the like is used. Recently, it is proposed to use a substrate made of an organic material such as polyethylene terephthalate (PET) which is lighter and more shock-resistant (refer to Japanese Unexamined Patent Application Nos. 8-186267, 10-144930, and 10-144931).
An organic material substrate has, however, a thermal expansion coefficient higher than that of a glass substrate or a silicon substrate. For example, as shown in FIG. 14, when a functional layer 103 is crystallized by being irradiated with an energy beam, there are problems such that a substrate 101 expands by a heat conduction via a heat-resistant layer 102 to the substrate 101, a very large stress instantaneously works on the functional layer 103, a crack occurs and, in a worse case, peeling occurs. In the case of manufacturing a functional device by using the organic material substrate, therefore, sufficient characteristics and reliability cannot be obtained.
The invention has been achieved in consideration of the problems and its object is to provide a functional device having no crack and capable of displaying excellent functional characteristics and a method of manufacturing the same.
A functional device of the invention has a functional layer provided on one of surfaces of a substrate and comprises: a heat-resistant layer which is a single layer or constructed by a plurality of layers provided between the substrate and the functional layer; and a low-temperature softening layer provided between the heat-resistant layer and the substrate and having a softening temperature lower than that of the substrate.
Another functional device according to the invention comprises: a functional layer; a low-temperature softening layer provided on one of surfaces of the functional layer and having a softening temperature of 80xc2x0 C. or lower; and a heat-resistant layer which is a single layer or constructed by a plurality of layers provided between the low-temperature softening layer and the functional layer.
A method of manufacturing a functional device according to the invention, in which a functional layer is provided on a substrate, comprises the steps of forming a low-temperature softening layer having a softening temperature lower than that of a substrate on the substrate; forming a heat-resistant layer which is a single layer or constructed by a plurality of layers on the low-temperature softening layer; and forming the functional layer on the heat-resistant layer.
In the functional device and the method of manufacturing the same according to the invention, by the low-temperature softening layer provided between the substrate and the functional layer, a stress caused by thermal expansion of the substrate is absorbed, thereby enabling occurrence of a crack and peeling in the functional layer to be prevented.
Since the another functional device according to the invention comprises the low-temperature softening layer having a softening temperature of 80xc2x0 C. or lower, occurrence of a crack in the functional layer caused by a difference in thermal coefficient of expansion can be prevented.
Further, in the functional device and the method of manufacturing the same according to the invention, it is preferable to provide a warp suppression layer for suppressing a warp which occurs in association with thermal deformation of the substrate on the surface opposite to the surface on which the functional layer is provided on the substrate. The warp suppression layer may be a composite layer of a polymer layer made of an organic polymer and a heat-resistant layer which is a single layer or constructed by two or more layers. The warp suppression layer may be only a polymer layer made of an organic polymer.
Other and further objects, features and advantages of the invention will appear more fully form the following description.